Scope troubles with a swap function

Matthew D. Wood woodm at equire.com
Fri Aug 10 22:14:08 CEST 2001


Ok, so swap works, just like you said it should.  (I'm still mad that I 
didn't see that.)

however, the next project is to write a 'cycle_variables' function (or 
single line like you did before.)

So, this time, we take an arbitrary number of variables, and cycle them 
through.

def cycle (*vars) :
   print 'Cycling the variables...'
   vars = list(vars)
   print vars
   temp = vars[0]
   for index in range(len(vars) - 1) :
       vars[index] = vars[index + 1]

   vars[-1] = temp
   print vars
   print 'See, I cycled them!'

A = 'a'
B = 'b'
C = 'c'
D = 'd'

print """Here's the original set:"""
print (A, B, C, D)
print

cycle(A, B, C, D)

print
print (A, B, C, D)
print """Oh, I guess I didn't really cycle them afterall."""


Obviously, that example doesn't work.  Is there a way to make it work?  
I don't necessarily know if such a function would be all that usefull, 
but I can't sleep at night because I haven't been able to figure out the 
solution.  (That's a little exageration  :'-(   )

And once again, thanks for all you guys/gals who have helped me out.

Hans Nowak wrote:

>> ===== Original Message From "Matthew D. Wood" <woodm at equire.com> =====
>> Ok, I've been banging my head against this for a sufficiently long time
>> that I feel justified in presenting this personal challenge to the list.
>> 
>> How do you make a swap function?
> 
> 
> You don't need to. :)  Use:
> 
>   a, b = b, a
> 
>> How do you make a function or a callable object instance that will do
>> what the following code intends to do:
>> 
>>   def swap (heaven, hell) :
>>           purgatory = heaven
>>           heaven = hell
>>           hell = purgatory
> 
> 
> Python's internal workings are very different from languages like C and 
> Pascal. A Python "variable" is not a location in memory that you can fill with 
> arbitrary data. This Pascal code:
> 
>   X := 3;
>   X := 42;
> 
> changes the value of X, which resides at a certain location in memory. But the 
> equivalent Python code
> 
>   x = 3
>   x = 42
> 
> does not do exactly the same; rather, it binds the name 'x' to the value 3, 
> then rebinds that name to the value 42. Other people explain this better than 
> me; see
> 
>   http://w1.132.telia.com/~u13212494/guides/python-objects.htm
> 
> HTH,
> 
> --Hans Nowak
> 
> 
> 

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